The present invention generally relates to spinal stabilization systems and more specifically relates to pedicle screw assemblies and tooling used with pedicle screw assemblies.
The spinal column is a highly complex system of bones and connective tissues that provides support for the body and protects the delicate spinal column and nerves. The spinal column includes a series of vertebrae stacked one atop the other, whereby each vertebral body includes a relatively strong bone portion forming the outside surface of the body (cortical) and a relatively weak bone portion forming the center of the body (cancellous). Situated between each vertebral body is an intervertebral disc that provides for cushioning and dampening of compressive forces applied to the spinal column. The vertebral canal containing the delicate spinal cords and nerves is located just posterior to the vertebral bodies.
Various types of spinal column disorders are known and include scoliosis (abnormal lateral curvature of the spine), kyphosis (abnormal forward curvature of the spine, usually in the thoracic spine), excess lordosis (abnormal backward curvature of the spine, usually in the lumbar spine), spondylolisthesis (forward displacement of one vertebra over another, usually in a lumbar or cervical spine) and other disorders caused by abnormalities, disease or trauma, such as ruptured or slipped discs, degenerative disc disease, fractured vertebra, and the like. Patients that suffer from such conditions usually experience extreme and debilitating pain as well as diminished nerve function.
In order to correct or alleviate the above-mentioned disorders, spinal fixation procedures have been developed whereby surgical implants are used for fusing together and/or mechanically immobilizing adjacent vertebrae of the spine. Spinal fixation may also be used to alter the alignment of the adjacent vertebrae relative to one another so as to alter the overall alignment of the spine. Such techniques have been used effectively to treat the above-described conditions and, in most cases, to relieve pain suffered by the patient. However, as will be set forth in more detail below, there are some disadvantages associated with current fixation devices.
One particular spinal fixation procedure includes immobilizing the spine by using orthopedic rods, commonly referred to as spine rods or spinal stabilizing rods, that run generally parallel to the spine. This is accomplished by exposing the spine posteriorly and fastening bone screws to the pedicles of the appropriate vertebrae. The pedicle screws are generally placed two per vertebra, one at each pedicle on either side of the spinous process, and serve as anchor points for the spine rods. Clamping elements adapted for receiving a spine rod therethrough are then used to join the spine rods to the screws. The aligning influence of the rods forces the spine to conform to a more desirable shape. In certain instances, the spine rods may be bent to achieve the desired curvature of the spinal column.
U.S. Pat. No. 5,129,388 to Vignaud et al. discloses a spinal fixation device including a pedicle screw having a U-shaped head rigidly connected to the screw. The U-shaped head includes a U-shaped channel for receiving a spine rod therein. The U-shaped head is internally threaded so that a set screw having external threads may be screwed therein. After the pedicle screw has been inserted into bone and the spine rod is positioned in the U-shaped channel, the set screw is threaded into the internal threads of the coupling element for securing the spine rod in the U-shaped channel and blocking relative movement between the spine rod and the pedicle screw. The fixation device also includes a cap covering an upper portion of the U-shaped element to prevent the arms of the U-shaped element from spreading upon threading the set screw into the U-shaped head.
Surgeons have frequently encountered considerable difficulty when attempting to insert spinal fixation devices such as those disclosed in the above-mentioned '388 patent. For example, surgeons have frequently been unable to efficiently and adequately place the spine rod into the U-shaped heads of the bone screws. This is because the U-shaped heads of the screws are often not aligned with one another due to curvature in spines and the different orientations of the pedicles being instrumented. The spine rods are often bent in multiple planes in order to couple the pedicle screws to the rod, which may lead to weaker connections with the rod. These problems also result in significantly longer operations, thereby increasing the likelihood of complications associated with surgery.
In response to the problems noted in the '388 patent, U.S. Pat. No. 5,733,286 to Errico et al., U.S. Pat. No. 5,672,176 to Biedermann et al., and U.S. Pat. No. 5,476,464 to Metz-Stavenhagen disclose polyaxial spinal fixation devices wherein the anchoring element fixed to the bone has a spherically-shaped head. The fixation devices in the subject patents also have orthopedic rod capturing assemblies for securing orthopedic rods in the capturing assemblies and connecting the rods with the anchoring elements. The spherically shaped heads of the anchoring elements permit movement of the anchoring elements relative to the orthopedic rod capturing assemblies.
In spite of the above-mentioned devices, there remains a need for improved tools and techniques for inserting spine rods into the rod-capturing portions of pedicle screw assemblies. The also remains a need for improved spine rods that may be easily coupled with a pedicle screw assembly.